Antibiotic susceptibility of marine Planctomycetes

Antimicrobials are naturally produced by microbes and therefore have always been present in their environment, as well as accompanying resistance mechanisms. The antibiotic resistance profile of environmental species is particularly relevant since genetic determinants of resistance can spread through horizontal gene transfer and reach clinically important species. The phylum Planctomycetes comprises Gram-negative bacteria characterised by unusual features and appear to be ubiquitously distributed. Members of this group have recently been characterised as producers of bioactive compounds, namely antimicrobials, but their antibiotic susceptibility profile has been scarcely studied. In this study, the antibiotic susceptibility profile of six phylogenetically distinct strains of Planctomycetes was assessed. All strains showed resistance to beta-lactams, aminoglycosides and glycopeptides. Our results showed that antibiotics which target protein synthesis or DNA replication, with the exception of aminoglycosides, were the most effective against the tested strains. The highest efficacy was observed for chloramphenicol, clindamycin and ciprofloxacin. The highest level of antimicrobial resistance was observed in the uncharacterised novel taxon Planctomyces sp. strain FF15 which was only susceptible to erythromycin and ciprofloxacin.